1. Field of the Invention
The present invention relates to a method for preparing 1,3-dioxane compounds.
2. Description of the Background
1,3-dioxane compounds of the formula (I): ##STR3## wherein R.sup.1 and R.sup.2 are each, independently of the other, hydrogen or hydrocarbon radical; and X.sup.1 and X.sup.2 are identical or different electronegative groups; are presently used as intermediates for the preparation of UV stabilizer or X-ray contrast media. See, for example, EP-A2 0 220 034.
1,3-Dioxane compounds of the general formula I are usually prepared from the corresponding bis(hydroxymethyl)compounds II by acid-catalyzed acetalization or ketalization in accordance with the reaction equation: ##STR4## (see e.g. Mager, S.; Hopartean, I.; Horn, M.; Groso, I.; Stud. Univ., Babes-Bolyai, Ser.! Chem. 1979, 24(1), 23-8). The shift of equilibrium in this case is effected by azeotropic distillation, using a suitable entrainer, such as toluene or cyclohexane.
Conducting this method on an industrial scale presents problems, however, which predominantly result from the thermal sensitivity of the bis(hydroxymethyl) compounds. For example, the industrially important diethyl bis(hydroxymethyl)malonate decomposes from temperatures of as little as 50.degree. C. or higher, giving rise to numerous byproducts, inter alia formaldehyde (Welch, K. N.; J. Chem. Soc. London, 1930, 1). In the case of bis(hydroxymethyl) compounds containing cyano groups there is the further risk of toxic hydrogen cyanide being formed.
Thus, as expected, when industrially useful residence times are utilized, even if low-boiling entrainers such as toluene, cyclohexane or isopropyl acetate are used, product losses of up to 50% of the theoretical amount are observed, depending on the catalyst used.
On a laboratory scale it is possible to limit the bottom temperature to 50.degree. C., and thus avoid thermal decomposition reactions, simply by applying a vacuum. In this case, for example, the reaction of diethyl bis(hydroxymethyl)malonate with an equimolar amount of cyclohexanone in toluene, catalyzed by sulfuric acid, affords an isolated yield of &gt;85% of the theoretical yield. On an industrial scale, however, the condensation of the vapors entails very high investment and operating costs.
Thermal decomposition of the bis(hydroxymethyl) compounds under the reaction conditions of the acetalization or ketalization can also be avoided by the use of low-boiling entrainers (Eliel, E. L.; Banks, H. D.; J. Am. Chem. Soc. 94 (1972), 171). However, it has been found that low-boiling entrainers such as petroleum ether (30-60.degree. C.), methyl t-butyl ether or methyl acetate shift the above-described equilibrium only very slowly and, moreover, in the case of sterically hindered ketones not all the way. The achievable space-time yields are, therefore, entirely inadequate for industrial implementations. Further arguments against implementing this procedure on an industrial scale include the high flammability of low-boiling petroleum ethers (flash point &lt;20.degree. C.) and the usually low solubilities of the highly polar bis(hydroxymethyl) compounds.
Thus, a need exists for a method which affords a rapid and complete conversion of the bis(hydroxymethyl)compounds into the compounds of the formula (I) under conditions in which product losses due to thermal decomposition of the starting materials are virtually precluded.